<!doctype html> 
<html> 
<head> 
    <meta charset="utf-8"> 
    <title></title> 
</head>
<body>
<script type="text/javascript">
	
/*
var c = {
	'a':10,
	'b':20
}
*/
var a = [1,2,3,4];
a.b=5;
for(var i in a){
	//alert(i);//1,2,3,4
	alert(a[i])
	//0,1,2,3,b
}

</script>
</body>
</html>